The embodiments described herein relate generally to chemical analysis systems and, more particularly, to detecting chemicals using atmospheric pressure photoionization with direct analysis in real time and thermal desorption.
Most known ambient analysis methods and apparatus include a means to analyze chemical compositions directly from surfaces and objects without the need for sample preparation. There are many applications for ambient analysis in a wide range of fields such as toxic and non-toxic chemical compound contamination and sensing, pathogen and toxin diagnostics, environmental monitoring of pollutants, detection and control of chemical processes, and clinical analysis of urine and blood samples.
Some known ambient analysis methods and apparatus include a means to desorb and ionize analyte samples on surfaces and objects using a gaseous stream of metastable atoms such as helium, argon, or molecules such as nitrogen. The metastable gaseous stream is formed by passing the gases through a discharge region. The discharge also causes a heating of the gas, which may be supplemented with direct heating, and helps to desorb molecules from objects into the vapor phase, i.e., a flowing afterglow discharge, where they can then ionize by Penning ionization from the metastable gases. These resultant vapor phase ions can then be sampled by a mass spectrometer or other ion analysis method. This method is called direct analysis in real time (DART).
Another known ambient analysis method and apparatus is called desorption electrospray ionization (DESI), wherein an electrospray ionization (ESI) source is aimed at objects and surfaces that may have compounds adhered thereon. The ESI source impinges on the surface and a process of surface molecule ionization and ion desorption occurs. As with DART, these resultant vapor phase ions can then be sampled by a mass spectrometer or other ion analysis method.
Yet another known ambient analysis method and apparatus includes inserting a liquid or other sample into a sampling tube or capillary and then inserting the sample into a heating element. The sample can then vaporize and be ionized by a variety of atmospheric pressure ionization sources including an atmospheric pressure chemical ionization (APCI) source and/or photoionization including atmospheric pressure photoionization (APPI). Generally, photoionization produces a positively charged ion. This occurs because the absorption of a photon by a molecule can lead to dissociation of an electron. However, the generation of these electrons may also lead to negative ion formation by a number of possible mechanisms, such as electron attachment, dissociative electron attachment, and deprotonation. The generation of electrons can be enhanced by putting a high abundance of photoionizable compounds called dopants into the vapor phase. Dopants can also lead to an enhanced yield of positive ions since the positive dopant ions can react with neutral vapor molecules by charge transfer or proton transfer. Dopants may include, for example, toluene, benzene, chloro-benzene, acetone, anisole or a combination of these compounds or other compounds, which tend to be liquids and are then vaporized. Dopants may be used with methods that include ion mobility spectrometry, or ion mass spectrometry.
Each of these ambient analysis methods has specific benefits for specific applications. For example DART is most useful for rapidly screening large areas, whereas, in contrast, DESI is most useful for screening small areas, and therefore DESI is typically used to image the chemical compositions on surfaces and objects. However, these methods also have some key disadvantages that limit their applicability and utility. For example, the ionization methods of ESI and DART have limitations on the range of compounds that can be ionized, wherein non-polar molecules, such as petroleum compounds, polyaromatic hydrocarbons, pesticides, steroids, and lipids, may be weakly detected, or undetected altogether.
Also, ESI and DART ionization methods are based on, or at least are strongly dependent upon, ion-molecule type reactions, and are therefore susceptible to ion and matrix suppression due to competition for charge, particularly in those samples that are not prepared or cleaned up. DART does not operate well with air because the presence of oxygen degrades the discharge process needed to create a metastable gas flow. Therefore, to use DART, process gases must be purchased and stored. Further, DART tends to work best with helium gas. To reduce costs, DART may be adapted to use less expensive nitrogen gas. However, this leads to formation of metastable nitrogen molecules for ionization, wherein such metastable nitrogen is not nearly as effective as metastable helium.